1. Technical Field
The present invention relates to an ophthalmic apparatus for measuring the shape of the cornea of the eye of a subject.
2. Background Information
Conventionally, in an ophthalmic apparatus for precisely measuring the shape of the cornea, concentric light transmitting portions, that is, light transmitting ring patterns (a placido pattern) are provided to a flat pattern plate, and plural light sources are arranged along the light transmitting ring pattern on a holding plate behind the pattern plate. The light source emits infrared light. This is because infrared light does not cause pupil contraction of the eye of the subject which would occur if a light transmitting ring pattern were projected in visible light.
A filter is located between the pattern plate and the subject on a base material, such as a resin that transmits infrared light and blocks visible light, so that the subject will not see the light transmitting ring pattern projected on the cornea. This configuration is effective to prevent instrument myopia caused by seeing at the light transmitting ring pattern at a short distance. In addition to the ophthalmic apparatus for measuring the shape of the cornea having a filter to block visible light, an ophthalmic apparatus having a plurality of functions, such as an ophthalmic apparatus for measuring the shape of the cornea and the accommodation ability of the eye, and an ophthalmic apparatus for measuring the shape of the cornea and the aberration of optical properties of the eyeball, may also have a filter for blocking visible light.
A flat pattern plate is modified to have a cup shape or a cone shape so as to reduce the size of the ophthalmic apparatus. In the case of a pattern plate having a cup shape or a cone shape, when a filter having a flat shape is disposed between the pattern plate and a subject and is disposed in the vicinity of the subject, the filter may come into contact with the subject. Therefore, the filter is also formed into a cup shape or a cone shape.
In an ophthalmic apparatus having such a structure, light emitted from a light source is converted to a light flux having a shape of plural concentric light transmitting ring patterns, and the light flux is projected onto the cornea of the eye of a subject. Then, the shape of the reflected light of the cornea of the eye of the subject with the concentric light transmitting ring patterns is compared with a shape of the projected pattern so as to observe the difference, and the shape of the cornea is measured from the differences of the shapes (for example, see Japanese Patent No. 3497006).
In a conventional ophthalmic apparatus, a filter that transmits infrared light but blocks visible light is disposed between a pattern plate and a subject, whereby the pattern plate and the filter are required to have the same shape, and the structure is complicated. Moreover, a filter must be disposed between a pattern plate and a subject, and the working distance of an ophthalmic apparatus is thereby somewhat short.